How do I extend a Path Animation curve for a longer fly-through but keep the frame rate at 24 fps?
You want the curve to be longer, right? Sorry if I misunderstood that part :P

Providing that is the question:

Step 1: Highlight your Nurbs path by pressing right click

Step 2 Enter edit mode either by pressing TAB or by changing it in the 3D View Menu

Step 3: Select the point closest to the direction you would like to go in, then press E for extrude or hold down CTRL and click on the screen, this will add additional vertices to your path which will let the path be longer. 

If you have more questions do not be afraid to ask for help :)

EDIT: This fixed @kimholder's problem

Does the Frame Rate become slower or faster when you do this? I'm not sure what you are trying to achieve, but if it is becoming slower, in my experience it was the viewport not being able to keep up. If its becoming faster, well thats just odd LOL. I think for the timing, you will need to fiddle with keyframes, if it is a visual issue because you are doing viewport playback, try viewing the scene in Solid View or Wireframe View and disable as many other objects as possible

to disable objects, under the Scene View (Top right in regular Blender view) click the eye. Another idea that might be helpful is to move objects to another render layer, to do this highlight objects that aren't as important to your scene immediately and press M, then select one of the other layers and you will see a dot appear. You can quickly access these again and move them back as you wish.